Optical birefringence and x-ray measurements are reported for single crystals \((0.5 mm \times 2 mm \times 5 \mu m)\) of dipalmitoylphosphatidylcholine as a function of temperature and relative humidity. Large single crystals grown in \(CHCl_3\)/acetone at a low water concentration undergo an irreversible phase transformation at \(20^{\circ}C\) and 70% relative humidity or at \(60^{\circ}C\) and 65% relative humidity. After this transformation occurs, the crystals can be made to undergo a reversible transformation into and out of the \(L_{\alpha}\) phase. The x-ray-determined mosaic spread in the hydrated crystal is less than \(\approx 0.04^{\circ}\) both above and below the main transition.